Scene color balance algorithms employed in automatic printers conventionally employ measurements of film density (R,G,B) and related mathematical techniques to attempt to deduce from the film the optimum level of chromatic correction to be applied on a scene-by-scene basis to the exposure of the film to photographic print paper. It is known that scene color-induced printer correction failures occur when the printer algorithm is unable to differentiate between sources of color variability in the film image that require high levels of chromatic correction (e.g. non-daylight illuminants, film processing effects, film keeping effects and manufacturing variability) and source of color variability that do not require high levels of chromatic correction, (e.g. highly colorful objects in the scene). It is desirable, therefore, to be able to identify and segregate sources of color variability according to the level of chromatic correction required.
One of the most common causes of color variability in the film image is scene illuminant. It is useful to be able to identify the type of scene illuminant and input this information to the printer algorithm for use in selecting and applying the appropriate level of chromatic correction. In commonly assigned U.S. Pat. Nos. 4,827,119 and 5,037,198, camera apparatus is described which is capable of detecting and discriminating among the various common sources of scene illuminants: natural, tungsten and fluorescent, by performing a Fourier series analysis on one or more of the harmonics of the illuminant signal generated from a light sensor in the camera. It is possible to record this discriminated data at the time of image capture for transfer to the photofinishing printer to be applied to the exposure determining algorithm process. Such data recording transfer processes might be, for example, optical data recording as a latent image on the film, magnetic data recording either on a magnetic layer formed on the film or on separable magnetic media, or electronic recording on a programmable memory medium (EEPROM).
The employment of specialized illuminant detection and discrimination apparatus such as referred to above, while effective for the purpose, has the disadvantage of adding cost and complexity to the camera design. Moreover, such systems are based on detection of known harmonic frequencies and are therefore limited to existing sources of illuminant. New light sources generating different harmonic frequencies would require new detection apparatus thus obsoleting existing cameras. The added cost of performing Fourier series based analysis in the camera could well preclude its use in lower cost cameras thus limiting the general use of illuminant detection techniques for improved chromatic correction in printing processes.
In U.S. Pat. No. 5,016,039 there is described camera apparatus for sensing and recording certain scene illuminant information which is to be read and utilized at a photographic printer to control exposure of the film image onto print paper. The information recorded involves light level, flash fire and subject distance to determine color temperature of the scene illuminant. A complex analysis is performed in an attempt to discriminate precisely among different range of daylight-related color temperatures and to discriminate among different non-daylight illuminants. In a low light level (non-daylight) scene, when the subject distance exceeds the point at which flash is effective, the patent teaches the use of a color temperature meter to accurately determine the scene illuminant color temperature to be recorded and conveyed to the photographic printer. No mention is made of how the photographic printer algorithms make use of the recorded and conveyed information. Moreover, the apparatus disclosed involves complex calculations to finely discriminate among the various scene illuminants and light levels to record precise color temperature information. It also requires the use of a color temperature meter to achieve illuminant discrimination in all regions of the scene photospace. All of this raises the cost and complexity of the camera apparatus and does not address the problem of providing simplified algorithm operation of photographic printer exposure techniques.
In U.S. Pat. No. 5,168,303, techniques are described for determining control of photographic printer algorithms from data recorded at the time of picture taking. This patent discloses numerous methods for estimating, at the printer, the color temperature of the scene illuminant from data such as light value, flash fire, time and date of taking the picture and the geographical location where the picture was taken. This information is then used to control the printer exposure algorithms to automatically override the density-based exposure determinants, including chromatic correction, so as to preserve the color cast of the scene illuminant in the print (except for elimination of greenish cast caused by fluorescent illuminant, this being an added step requiring complex illuminant detection and/or algorithm manipulation). The effect achieved by the disclosure of this patent is the opposite of the objective of the present invention which is to provide a simple method of removing the color cast created by non-daylight scene illuminant from the printed image.
It is therefore an object of the present invention to provide a simplified photographic system for detecting and discriminating scene illuminants for use in setting appropriate levels of chromatic correction in the photographic printing process.
It is another objective of the invention to achieve appropriate chromatic correction at a photographic printer by a simplified inference as to the scene illuminant that does not require complex discrimination of many different levels and types of scene illuminant.
It is a still further object of the invention to provide simplified illuminant detection and discrimination for control of automatic printer chromatic correction algorithms that utilizes data derived from existing conventional sensor apparatus typically found in the camera.